Major problems exist in producing oil from heavy oil reservoirs due to the high viscosity of the oil. Because of this high viscosity, a high pressure gradient builds up around the well bore, often utilizing almost two-thirds of the reservoir pressure in the immediate vicinity of the well bore. Furthermore, as the heavy oils progress inwardly to the well bore, gas in solution evolves more rapidly into the well bore. Since gas dissolved in oil reduces its viscosity, this further increases the viscosity of the oil in the immediate vicinity of the well bore. Such viscosity effects, especially near the well bore, impede production; the resulting wasteful use of reservoir pressure can reduce the overall primary recovery from such reservoirs.
Similarly, in light oil deposits, dissolved paraffin in the oil tends to accumulate around the well bore, particularly in screens and perforations and in the deposit within a few feet from the well bore. This precipitation effect is also caused by the evolution of gases and volatiles as the oil progresses into the vicinity of the well bore, thereby decreasing the solubility of paraffins and causing them to precipitate. Also, the evolution of gases causes an auto-refrigeration effect which reduces the temperature, thereby decreasing solubility of the paraffins. Similar to paraffin, other condensable constituents also plug up, coagulate or precipitate near the well bore. These constituents may include gas hydrates, asphaltenes and sulfur. In certain gas wells, liquid distillates can accumulate in the immediate vicinity of the well bore, which also reduces the relative permeability and causes a similar impediment to flow. In such cases, accumulations near the well bore reduce the production rate and reduce the ultimate primary recovery.
Electrical resistance heating has been employed to heat the reservoir in the immediate vicinity of the well bore. Basic systems are described in Bridges U.S. Pat. No. 4,524,827 and in Bridges et al. U.S. Pat. No. 4,821,798. Tests employing systems similar to those described in the aforementioned patents have demonstrated flow increases in the range of 200% to 400%.
A major engineering difficulty is to design a system such that electrical power can be delivered reliably, efficiently, and economically down hole to heat the reservoir. Various proposals over the years have been made to use electrical energy in a power frequency band such as DC or 60 Hz AC, or in the short wave band ranging from 100 kHz to 100 MHz, or in the microwave band using frequencies ranging from 900 MHz to 10 GHz. Various down hole electrical applicators have been suggested; these may be classified as monopoles, dipoles, or arrays of antennas. A monopole is defined as a vertical electrode whose size is somewhat smaller than the thickness (depth) of the deposit; the return electrode is usually large and is usually placed at a distance remote from the deposit. For a dipole, two vertical electrodes are used and the combined extent is smaller than the thickness of the deposit. These electrodes are excited with a voltage applied to one with respect to the other.
Where heating above the vaporization point of water is not needed, use of frequencies significantly above the power frequency band is not advisable. Most typical deposits are moist and rather highly conducting; high conductivity increases the lossiness of the deposits and restricts the depth of penetration for frequencies significantly above the power frequency band. Furthermore, use of frequencies above the power frequency band may require the use of expensive radio frequency power sources and coaxial cable or waveguide power delivery systems.
An example of a power delivery system employing DC to energize a monopole is given in Bergh U.S. Pat. No. 3,878,312. A DC source supplies power to a cable which penetrates the wellhead and which is attached to the production tubing. The cable conductor ultimately energizes an exposed electrode in the deposit. Power is injected into the deposit and presumably returns to an electrode near the surface of the deposit in the general vicinity of the oil field. The major difficulty with this approach is the electrolytic corrosion effects associated with the use of direct current.
Hugh Gill, in an article entitled, "The Electro-Thermic System for Enhancing Oil Recovery," in the Journal of Microwave Power, 1983, described a different concept of applying power to an exposed monopole-type electrode in the pay zone of a heavy oil reservoir. In his FIG. 1 Gill shows a schematic diagram wherein electrically isolated production tubing replaces the electrical cable used in the Bergh patent. The current flows from the energizing source down the production tubing to the electrode, and then returns to an electrode near the surface to complete the electrical circuit. The major difficulty with this involves two problems. First, the production casing of the well surrounds the current flowing on the tubing. In such instances, the current itself produces a circumferential magnetic field intensity which causes a large circumferential magnetic flux density in the steel well casing. Under conditions of reasonable current flow to the electrode this high flux density causes eddy currents and hysteresis losses in the casing. Such losses can absorb most of the power intended to be delivered down hole into the reservoir. The second major problem is associated with the skin effect losses in the production tubing itself. While the DC resistance of the tubing is small, the AC resistance can be quite high due to the skin effect phenomena caused by the circumferential magnetic field intensity. This generates a flux and causes eddy currents to flow. The eddy currents cause the current to flow largely on the skin of the production tubing, thereby significantly increasing its effective resistance. Such problems are minimal in the system of the Bergh patent, wherein the DC current avoids the problems associated with eddy currents and hysteresis losses.
Another method to partially mitigate the hysteresis losses in the production casing is described by William G. Gill in U.S. Pat. No. 3,547,193. In this instance the production tubing, typically made from steel, is used as one conductor to carry current to an exposed monopole electrode located in the pay zone of the deposit. Current flows outwardly from the electrode and then is collected by the much larger well casing. As implied in this patent, the design is such as to force the current to flow on the inside of the production casing, and thereby reduce by about 50% the eddy currents and hysteresis losses associated with the production casing.
Power delivery systems for implanted dipoles in the deposits have largely employed the use of coaxial cables to deliver the power. For example, in U.S. Pat. No. 4,508,168 by Vernon L. Heeren, a coaxial cable power delivery system is described wherein one element of the dipole is connected to the outer conductor of the coaxial cable and the other to the inner conductor. Heeren suggests the use of steel as a material for the coaxial transmission line which supplies RF energy to the dipole. However, it is more common practice to use copper and aluminum as the conducting material. Unfortunately, both copper and aluminum may be susceptible to excessive corrosion in the hostile atmosphere of an oil well. This produces a dilemma, inasmuch as aluminum and copper cables are much more efficient than steel for power transmission but are more susceptible to corrosion and other types of degradation.
Haagensen, in U.S. Pat. No. 4,620,593, describes another method of employing coaxial cables or waveguides to deliver power to down hole antennas. In this instance, the coaxial cable is attached to the production tubing and results in an eccentric relationship with respect to the concentric location of the pump rod, the production tubing and the production casing. Haagensen's object is to use the coaxial cable as a wave guide to deliver power to antenna radiators embedded in the pay zone of the deposit. However, as stated previously, energy efficient materials for the wave guides or cables are usually formed from copper or aluminum, and these are susceptible to corrosion in the environment of an oil well. The conversion of AC power frequency energy into microwave energy is costly. The cables themselves, when properly designed to withstand the hostile environment of an oil well, are also quite costly. Furthermore, it appears unlikely that the microwave heating will have any significant reach into the oil deposit and the heating effects may be limited to the immediate vicinity of the well bore.
To address some of these difficulties Bridges et al., in U.S. Pat. No. 5,070,533, describes a power delivery system which utilizes an armored cable to deliver AC power from the surface to an exposed monopole electrode. In this case, an armored cable which is commonly used to supply three-phase power to down hole pump motors is used. However, the three phase conductors are conductively tied together and thereby form, in effect, a single conductor. From an above ground source, the power passes through the wellhead and down this cable to energize an electrode imbedded in the pay zone of the deposit. The current then returns to the well casing and flows on the inside surface of the casing back to the surface. The three conductors in the armored cable are copper. The skin effect energy loss associated with using the steel production tubing as the principal conductor is thereby eliminated. However, several difficulties remain. A low frequency source must be utilized to overcome the hysteresis and eddy current losses associated with the return current path through the steel production casing. Furthermore, non-magnetic armor must be used rather than galvanized steel armor. Galvanized steel armor that surrounds the downward current flow paths on the three conductors causes a circumferential magnetic flux in the armor. This circumferential flux can create significant eddy currents and hysteresis losses in the steel armor and may result in excessive heating of the cable. As a consequence, in order to avoid the excessive heating problems and losses, Monel armor is used, which is more expensive than galvanized steel armor. However, a major benefit of the approach described in Bridges et al. U.S. Pat. No. 5,070,533 is that commonly used oil field components are used throughout the system, with the exception of the apparatus in the immediate vicinity of the pay zone. Offsetting these benefits are the high cost of cable using Monel armor and the need to use a frequency converter which converts 60 Hz AC power to frequencies between 5 Hz and 15 Hz.
Another problem occurs in the case of horizontal oil wells. Typically, the boring tool is deviated such that a long horizontal borehole is formed in the oil reservoir. The well is then completed by installing a perforated casing or screen almost the entire length of the horizontal borehole. Such horizontal completions often are more than several hundred meters in length. In some reservoirs production could be greatly enhanced by the use of electrical heating. Because the spreading resistance of the electrode is inversely proportional to its length, the "electrode resistance", instead of being one to ten ohms as in the case of a vertical well, may be considerably smaller than one ohm, and could be smaller than the series resistance of the cable or tubing used to deliver power from the wellhead to the reservoir. When this occurs, most of the heating power is expended in the cable or tubing and not in the deposit. Another problem is that the flow rate from horizontal wells is quite large and substantial amounts of power, possibly in the order of several hundred kilowatts, may be expended in the deposit to obtain the full benefit of near-well bore electrical heating of the deposits for a horizontal completion.